The present invention relates to the field of digital communications.
A conventional digital transmission scheme is considered (FIG. 1): the signal received, consisting of a sequence of symbols, originates from a digital modulator 1, then may be deformed by a propagation channel 2.
In many systems the receiver 3 requires two time synchronisation data signals: the xe2x80x9cdatexe2x80x9d of the boundaries between symbols (for the operation of the demodulator), which supplies a synchronisation data signal modulo the duration of a symbol; and a system xe2x80x9cclockxe2x80x9d, whose accuracy is in whole numbers of symbol time, e.g. taking the form of a boundary between xe2x80x9cframesxe2x80x9d if the transmitted signal has this structure (to take advantage, for example, of the presence of learning sequences at precise instants, and/or to know where to go to subsequently decode the desired data). This clock is indicated by one or more special symbols which act as markers, which the receiver must detect.
Furthermore, the signal received must be correctly situated in the frequency domain, expected by the demodulator 4 of the receiver 3. But since the oscillators of the receiver and the transmitter have a limited accuracy, we may expect a frequency deviation which should be corrected.
The synchronisation component 5 shown in diagram form in FIG. 1 is first called upon by the receiver. It will supply the two data signals (time and frequency). The time data will then be used directly by the demodulator/decoder 4, which will shift its references to obtain both the correct position of the symbols and the useful information. The frequency data will be used by a correction circuit 6 to analogically or digitally correct the frequency position of the signal received. The component 5 implements an algorithm whose object is to estimate the frequency deviation and to retrieve the time structure of the data.
The first important performance criterion of synchronisation is the reliability of the time and frequency data obtained. A second criterion is the cost necessary for obtaining these data (execution time, memory and computing power necessary). There are several categories of synchronisation methods:
the blind methods, currently called NDA (xe2x80x9cNon Data Aidedxe2x80x9d), which do not require the receiver to know part of the information transmitted (most often thanks to the use of properties of the transmitted signal). The majority of these methods, whether for estimating frequency or time difference, are based on the signal passing through non-linearities, causing lines which can be used for synchronising (see J. B. Anderson et al., xe2x80x9cDigital Phase Modulationxe2x80x9d, Plenum Press, 1986). Moreover, there is a family of analog solutions (PLL, Costas loop, etc, see J. C. Bic et al., xe2x80x9cElxc3xa9ments de communications numxc3xa9riquesxe2x80x9d [xe2x80x9cElements of Digital Communicationsxe2x80x9d], Volume 1, CNET/ENST Collection, Dunod, 1986). The drawback is that these techniques are specialised by type of modulation, and do not provide a system clock of a higher level than the symbol time;
for system synchronisation, solutions based on codes (especially convolutional codes) having good autocorrelation properties. Of course, this requires previous frequency and time synchronisation at the symbol level.
the methods based on a known frame (DA, xe2x80x9cData Aidedxe2x80x9d), where the receiver knows all or part of a frame, using it to estimate the synchronisation data. In the case of the GSM cellular radio communication system, a specific frequency, i.e. a purely sinusoidal signal, is used for frequency synchronisation, then a dedicated sequence (burst) enables time synchronisation. Another example is described in FR-A-2 745 134.
DA methods have the drawback of lowering the useful throughput, since part of the signal does not carry any information properly speaking. It should, however, be noted that NDA methods are not always applicable depending on the nature of the signal, i.e. that of the transmission encoder. The methods are thus very specific. In particular, there are many methods (see for example: A. D""Andrea et al., xe2x80x9cA Digital Approach to Clock Recovery in Generalized Minimum Shift Keyingxe2x80x9d, IEEE Transactions on Vehicular Technology, Vol. 39, No. 3, August 1990; A. D""Andrea et al., xe2x80x9cFrequency Detectors for CPM Signalsxe2x80x9d, IEEE Transactions on Communications, Vol. 43, No. 2-3-4, February/March/April 1995) applicable when the encoder is a CPM (xe2x80x9cContinuous Phase Modulationxe2x80x9d) modulator, which do not apply to other types of modulation. Moreover, their execution time performances, measured by the length of time for arriving at reliable synchronisation data, are sometimes poor. DA methods may prove more efficient in terms of execution time, for the same reliability, for a negligible reduction in useful throughput. In addition, they are completely general, and do not depend on the nature of the signal used.
There is therefore a need for a synchronisation process supplying, in a relatively short time, the time and frequency position information of the data. This process must operate for relatively large frequency and/or time differences, within the limit of the bandwidth and for any arrival time of the effective signal. The loss of useful information associated with the synchronisation execution time and/or with the insertion of known data must be negligible in relation to the useful throughput. In addition, it is desirable that this process should not be too costly in computing time and therefore basically use simple operations (addition, multiplication).
The usual method for achieving frequency synchronisation in the case CPM modulations of index k/p (J. B. Anderson et al., xe2x80x9cDigital Phase Modulationxe2x80x9d, Plenum Press, 1986) is a blind method (NDA) consisting of calculating the Fourier transform of the received baseband signal raised to the power p, and detecting lines in the spectrum thus obtained. Frequency lines xcex94f+i/(2Ts) appear in principle in this spectrum, xcex94f being the frequency deviation to be estimated, Ts the duration of a symbol and i an integer. But their detection is sometimes problematic: they can be embedded in the power spectral density of the signal at power p or in the noise picked up by the receiver, and there may be uncertainty over the valid integer i for a detected line. This depends on the characteristics of the CPM used: NDA methods are sometimes unusable.
The object of the present invention is to meet the aforementioned need, especially regarding the aspects of frequency synchronisation. The applications which are especially targeted are digital communication receivers needing a synchronisation component for initiating the decoding of data, for which NDA methods are impossible or not very effective.
According to the invention, a synchronisation method for a communication receiver is proposed, comprising the steps of evaluating a deviation between a modulation frequency, combined with a first baseband signal to form a signal transmitted on a propagation channel and a frequency used by the receiver to form a second baseband signal from a signal received on the propagation channel, and adjusting the frequency used by the receiver according to the evaluated deviation. Knowing a time segment of the first baseband signal and the time position of a corresponding segment of the second baseband signal, said segments comprising N samples at a sampling frequency Fe, a frequency transform Y(f), of size N, of the product of the complex conjugate of said segment of the first baseband signal by the corresponding segment of the second baseband signal is calculated, and said frequency deviation xcex94f is evaluated as being that for which the frequency transform Y(f) is the closest to C.Sc(fxe2x88x92xcex94f), where Sc(f) is the function                     S        c            ⁡              (        f        )              -                  1        -                  exp          ⁡                      (                                          -                2                            ⁢              j              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              Nf                /                                  F                  e                                                      )                                      1        -                  exp          ⁡                      (                                          -                2                            ⁢              j              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                /                Fe                                      )                                ,
and C is a complex coefficient.
In a particular embodiment, the step of evaluating said frequency deviation comprises:
determining a frequency fmax which maximises the quantity |Y(f)|2 among the N points of the frequency transform;
dividing the interval [fmaxxe2x88x92Fe/N, fmax+Fe/N] into a plurality of sub-intervals;
a dichotomic search in each of the sub-intervals to identify each frequency xcex4f which cancels out the quantity:                               F          ⁡                      (                          δ              ⁢                              xe2x80x83                            ⁢              f                        )                          =                  Re          ⁢                      {                                                            ⟨                                                            Y                      ⁡                                              (                        f                        )                                                              ,                                                                  S                        c                                            ⁡                                              (                                                  f                          -                                                      δ                            ⁢                                                          xe2x80x83                                                        ⁢                            f                                                                          )                                                                              ⟩                                *                            ·                              ⟨                                                      Y                    ⁡                                          (                      f                      )                                                        ,                                                            ∂                                                                        S                          c                                                ⁡                                                  (                                                      f                            -                                                          δ                              ⁢                                                              xe2x80x83                                                            ⁢                              f                                                                                )                                                                                                            ∂                                              (                                                  δ                          ⁢                                                      xe2x80x83                                                    ⁢                          f                                                )                                                                                            ⟩                                      }                                              (        1        )            
xe2x80x83where Re(.) and (.)* designate the real and the conjugate parts of a complex number and, for two complex functions of the frequency A(f) and B(f),  less than A(f),B(fxe2x88x92xcex4f) greater than  designates the inner product:                               ⟨                                    A              ⁡                              (                f                )                                      ,                          B              ⁡                              (                                  f                  -                                      δ                    ⁢                                          xe2x80x83                                        ⁢                    f                                                  )                                              ⟩                =                              ∑                          i              =              0                                      N              -              1                                ⁢                      xe2x80x83                    ⁢                                    A              ⁡                              (                                  f                  i                                )                                      ⁢                                          B                ⁡                                  (                                                            f                      i                                        -                                          δ                      ⁢                                              xe2x80x83                                            ⁢                      f                                                        )                                            *                                                          (        2        )            
xe2x80x83fi designating the point of rank i of the frequency transform; and
selecting the frequency deviation xcex94f as being that of the frequencies xcex4f which cancels out the quantity F(xcex4f) and for which the cost function:                               K          ⁡                      (                          δ              ⁢                              xe2x80x83                            ⁢              f                        )                          =                  "LeftDoubleBracketingBar"                                    Y              ⁡                              (                f                )                                      ⁢                                                            ⟨                                                            Y                      ⁡                                              (                        f                        )                                                              ,                                                                  S                        c                                            ⁡                                              (                                                  f                          -                                                      δ                            ⁢                                                          xe2x80x83                                                        ⁢                            f                                                                          )                                                                              ⟩                                ,                                                      S                    c                                    ⁡                                      (                                          f                      ⁢                                              xe2x80x83                                            ⁢                      δ                      ⁢                                              xe2x80x83                                            ⁢                      f                                        )                                                                                                "LeftDoubleBracketingBar"                                                            S                      c                                        ⁡                                          (                                              f                        -                                                  δ                          ⁢                                                      xe2x80x83                                                    ⁢                          f                                                                    )                                                        "RightDoubleBracketingBar"                                2                                              "RightDoubleBracketingBar"                                    (        3        )            
xe2x80x83is minimal, ∥.∥ being the norm associated with said inner product.
Said time segment of the first baseband signal may not be known a priori by the receiver. When the receiver is synchronised in time and frequency and it is desired to update the frequency synchronisation for the possible adjustment of a slight drift of the local oscillator, we can thus use the result of the demodulation and/or decoding as the xe2x80x9ctime segment of the first baseband signalxe2x80x9d to implement the procedure.
Quite often, said time segment of the first baseband signal will, however, be known a priori by the receiver, the method including a time synchronisation phase for obtaining the time position of said segment of the second baseband signal.
Frequency synchronisation then makes use of the signal segments required for time synchronisation, enabling the shortcomings of the NDA methods generally used to be overcome. These segments may remain moderate in size so that the implementation of the method has a moderate bandwidth impact. This size is equal to or greater than the aforementioned number N. Frequency synchronisation is thus dependent upon the success of time synchronisation, which is not too punitive if the probability of successful time synchronisation is high.
In a preferred embodiment, the time segment known a priori includes several consecutive occurrences of a first pattern of Lc samples, and possibly a second pattern longer than the first. The time synchronisation phase includes a modulo Lc estimating stage by maximising the correlation between the second baseband signal and the consecutive occurrences of the first pattern, and a modulo Lc ambiguity removing stage, providing a time synchronisation at the resolution of the samples by maximising the correlation between the second baseband signal and the second pattern.
After thus carrying out a first time synchronisation producing a whole number nest representing a time difference, in number of samples, of the second baseband signal with respect to said time segment of the first baseband signal, the first time synchronisation is refined by adding to the whole number nest the quantity:                               Δ          ⁢                      xe2x80x83                    ⁢          τ                -        1        -                  λ          ⁢                                    Re              ⁢                              xe2x80x83                            ⁢                              {                                                      ∑                                          q                      =                      0                                                                                      N                        c                                            -                      1                                                        ⁢                                      [                                          xe2x80x83                                        ⁢                                                                  (                                                                              ∑                                                          k                              =                              0                                                                                                                      L                                Sc                                                            -                              1                                                                                ⁢                                                      xe2x80x83                                                    ⁢                                                                                    r                                                                                                n                                  est                                                                +                                                                  q                                  ·                                                                      L                                    c                                                                                                  +                                k                                                                                      ·                                                          xe2x80x83                                                        ⁢                                                          x                              k                                                              c                                *                                                                                                                                    )                                            ·                                              xe2x80x83                                            ⁢                                              (                                                                              ∑                                                          k                              =                              0                                                                                                                      L                                Sc                                                            -                              2                                                                                ⁢                                                      xe2x80x83                                                    ⁢                                                                                    r                                                                                                n                                  est                                                                +                                                                  q                                  ·                                                                      L                                    c                                                                                                  +                                k                                                                                      ·                                                          xe2x80x83                                                        ⁢                                                          ⅆ                                                              x                                k                                                                  c                                  *                                                                                                                                                                    ⁢                                                  xe2x80x83                                                )                                                              ⁢                                          xe2x80x83                                        }                                                                                                      ∑                                  q                  =                  0                                                                      N                    c                                    -                  1                                            ⁢                                                "LeftDoubleBracketingBar"                                                            ∑                                              k                        =                        0                                                                                              L                          Sc                                                ⁢                        1                                                              ⁢                                                                  r                                                                              n                            est                                                    +                                                      q                            ·                                                          L                              c                                                                                +                          k                                                                    ·                                              x                        k                                                  c                          *                                                                                                      "RightDoubleBracketingBar"                                2                                                                        (        4        )            
where xcex is the predetermined normalisation factor:                     λ        =                                            (                                                ∑                                      k                    =                    0                                                                              L                      Sc                                        ⁢                    1                                                  ⁢                                                      "LeftBracketingBar"                                          x                      k                      c                                        "RightBracketingBar"                                    2                                            )                        2                                                              (                                                      ∑                                          k                      =                      0                                                                                      L                        Sc                                            -                      1                                                        ⁢                                                            "LeftBracketingBar"                                              x                        k                        c                                            "RightBracketingBar"                                        2                                                  )                            ·                              (                                                      ∑                                          k                      -                      0                                                                                      L                        Sc                                            -                      1                                                        ⁢                                                            "LeftBracketingBar"                                              ⅆ                                                  x                          k                          c                                                                    "RightBracketingBar"                                        2                                                  )                                      -                                          [                                  lm                  ⁡                                      (                                                                  ∑                                                  k                          =                          0                                                                                                      L                            Sc                                                    -                          1                                                                    ⁢                                                                        x                          k                          c                                                ·                                                  ⅆ                                                      x                            k                                                          c                              *                                                                                                                                            )                                                  ]                            2                                                          (        5        )            
Nc is the number of consecutive occurrences of the first pattern, LSc is the number of samples of the repetitive sequence formed by the consecutive occurrences of the first pattern in said time segment of the first baseband signal, rnest+q.Lc+k is the sample of rank nest+q.Lc+k of the second baseband signal, xkc is the sample of rank k of said repetitive sequence, dxkc is the sample of rank k of a sequence formed by the time derivative of said repetitive sequence, and Im(.) designates the imaginary part of a complex number.
Another aspect of the present invention relates to a synchronisation device for a communication receiver, comprising means of analysing a received signal, arranged for implementing a method such as that defined above.